A fluid bearing, i.e. an oil bearing or a gas bearing, and in particular a hydrodynamic bearing, tends to transmit a force to its casing which is proportional firstly to the stiffness of the bearing and secondly to its eccentricity, i.e. the distance between the inertial axis of the rotor and the bearing axis. Regardless of how well the rotor is balanced, the off-balance cannot be completely eliminated. The force transmitted to the casing thus tends to cause the casing to vibrate in a manner which in many cases is detrimental to the environment in which the rotary machine is located.
To remedy this drawback, proposals have already been made to compensate the forces exerted on the casing of a machine by a hydrodynamic bearing by using electrodynamic excitors which exert active damping on the casing tending to reduce the effect of the forces exerted by the hydrodynamic bearing. Such a device which acts directly on the structure to be stabilized is not entirely satisfactory since its effect cannot avoid residual vibration and the rotating forces due to the off-balance mass applied to the casing by the hydrodynamic bearing, which forces are applied not only at the frequency rotation of the rotor, but also at frequencies which are multiples thereof, thereby making compensation all the more difficult.
U.S. Pat. No. 4,626,754 describes a device for reducing vibration in rotary machines applicable to the case where the rotor is mounted on the casing by means of an active magnetic suspension including at least one radial magnetic bearing. In this device, vibration detectors, disposed on the casing serve to reduce the vibration to which the casing is subjected by acting on the servocontrol circuits of the active magnetic bearings. Such an active magnetic suspension of the rotor could thus be juxtaposed with a fluid bearing suspension in order to compensate the disturbing forces set-up by the fluid bearing by means of the negative stiffness provided by the magnetic bearing. However, this solution suffers from drawbacks due to the fact that it cannot prevent parasitic couples appearing on the stator due to the juxtaposition of the magnetic bearings and the fluid bearings in axially offset positions, and above all due to the fact that it requires powerful magnetic bearings capable of compensating the force required to cause the rotor to rotate about its inertia axis rather than about the axis of the fluid bearing, and not just compensating the centrifugal force due to the out-of-balance mass, with the necessary compensation force being proportional to the distance between the two axes and to the stiffness of the fluid bearing, which stiffness is always very high. Further, it is necessary to compensate for harmonics that are likely to appear because of the dynamic load on the fluid bearing.
The present invention seeks to remedy the drawbacks of prior art devices and to effectively reduce or even eliminate the vibration that may be applied to the casing of a rotary machine by virtue of the machine's rotor being supported by a fluid bearing.